Among the many types of pressure relief valves that have been developed is a simple and inexpensive valve having elements disposed on opposite sides of a valve opening or seat connected together by a stem on one inflexible part extending through the valve opening and having an enlarged head fitting into an opening in a relatively flexible valve disc so that spring loading of the first part seats the valve disc. Such a valve structure is shown in German Pat. No. 25 25 708 which corresponds to U.S. Pat. No. 4,056,121; however, it has been found that it is quite difficult to accurately determine the relative pressures for valve opening and also the opening pressure changes with time and valve use. The foregoing difficulties arise from the necessarily flexible nature of the valve disc mating with the valve seat. Deflection of the valve disc is only inaccurately predictable and furthermore will change with fatigue of the material so that spring loading pressure varies an indeterminate amount.
Valves of the type noted above are advantageous in their simplicity and the present invention provides a valve of this general type with no increase in complexity but without the noted limitations.